The present invention relates to an absolute position detecting method and an apparatus therefor.
Conventional absolute-type encoders capable of detecting an absolute position generally include a light source, a code plate having binary code patterns formed on a plurality of tracks arranged along the direction of movement and a plurality of photosensors arranged in correspondence to the tracks, thereby detecting the output signals of the photosensors to determine the position of the code plate. Also, the binary code patterns include pure binary code patterns capable of producing binary number position data and Gray code patterns designed so that the binary code patterns of different orders or position differ in boundary position from one another so as to prevent the position data from being disturbed considerably due to minor forming errors among the binary code patterns of different orders.
In addition, there has been known another absolute-type encoder employing a combination of a detection method using the previously mentioned binary code patterns and an incremental-type detection method so that a rough absolute position within a given detection range is detected by means of the binary code patterns and its infinitesimal position within the detection range is detected from a reference position established within the detection range by the incremental method.
Also, there has been known by JP-A-61-189415 an absolute position detecting apparatus including an absolute position detector using binary code patterns and a high-resolution detector for detecting its position within a minimum detection range by phase division.
Then, with the position detecting method employing the binary code patterns, while the detection of an absolute position can be effected digitally so that it is not liable to suffer the effect of vibration or noise and the position detection is effected with a high degree of accuracy, to effect a high-resolution position detection requires that the binary code patterns of varying orders themselves must be formed with a very high degree of accuracy and also the relative positions of these varying-order binary code patterns must be determined with a very high degree of accuracy. In the case of the previously mentioned pure binary code patterns, the relative positions of the binary code patterns of varying orders must be in complete coincidence, and also in the case of the Gray code patterns, all the binary code patterns of varying orders must be in coincidence with an accuracy of one fourth the bits in the lowest-order binary code pattern. Thus, these systems have disadvantages in that the forming accuracy of the patterns and the interpattern forming accuracy must be so high that position detection with a high resolution on the order of 2.sup.15 to 2.sup.20 cannot be realized due to technical manufacturing difficulties except special cases where huge costs, resulting from the initial manufacturing requirements, can be borne. Another disadvantage exists in that to maintain a high degree of signal detection accuracy for all the orders requires complicated signal processing capability thus increasing costs and making the provision of high speed response difficult.
Also, while the system combining the position detecting method employing binary code patterns and the incremental-type position detecting method has the effect of requiring relatively small numbers of patterns and photosensors thus making it possible to inexpensively effect a high accuracy position detection, there is a disadvantage in that it is necessary to perform the operation of detecting a rough position by means of the binary code patterns, searching a reference signal pattern to detect the reference position established in its detection range and incrementally detect the amount of displacement from the reference position to a position to be detected and that this operation is complicated thus decreasing the response speed.
In the case of a system simply combining the absolute position detector and a high-resolution detector by phase division, there is not only a disadvantage in that an increase in the detection range results in an increase in the slit pitch of the high-resolution detector thus causing a deterioration in the detection accuracy by phase division, but also there is another disadvantage in that if the boundaries of the detection positions in the higher-order position detector are not in coincidence with the boundaries of the detection ranges in the high-resolution detector with a greater degree of accuracy than the accuracy required for the position detection, errors are caused in the detection data at these boundary portions.